The following photocatalytic reaction is well-known as a technique to obtain an aimed subject. That is, the light is irradiated to a solid compound which has a photocatalytic function so as to generate exited electrons and holes. Then a substance is oxidized or reduced by said exited electrons and holes and obtain the aimed object.
In particular, photocatalytic decomposition of water, is interested in from the view point of photo energy transformation. Further, a photocatalyst which shows activity to the photo decomposing reaction of water can be recognized as an excellent photo functional material which provides function such as photo absorption, electrolytic separation or oxidation-reduction reaction at the surface.
Kudo, Kato et al, are explaining that alkaline tantalate or alkaline earth element is a photocatalyst which shows excellent activity to the stoichiometric photocatalytic decomposing reaction of water by quoting various prior arts [for example, Catal. Lett., 58(1999), 153-155; Chem. Lett., (1999), 1207; Surface, Vol.36, No.12 (1998), 625-645 (hereinafter shortened to Document A)]. In above mentioned Document A, there is an explanation about an useful photocatalytic materials for proceeding the decomposing reaction of water to hydrogen and/or oxygen using a photocatalyst, and many indications about photocatalyst used for stoichiometric photocatalytic decomposing reaction of water are mentioned.
Further, a photocatalyst which loads a promoter such as platinum or NiO is referred.
However, the photocatalysts explained in these documents are mainly the compound containing oxygen as a non-metallic element and a transition metal such as Ta, Nb and Ti as a metal. And in the cases of various solid photocatalyst, since the width of a forbidden band exists between a valence electron band and a conduction band, that is, band gap energy is larger than 3 eV, it is difficult to excite it by low energy under 3 eV, namely it is difficult to generate electrons and positive holes by light. On the contrary, almost all of the conventional solid photocatalyst which can generate electrons and holes by visible-light irradiation are unstable under the condition of photocatalytic water decomposing reaction. For example, the band gap energy of CdS or Cu—ZnS is 2.4 eV, but the catalytic reaction is restricted because it is affected by photo-corrosive action, which is corrosive oxidative action. In the meanwhile, almost all of the solar light which reaches to the surface of the earth is the visible light radiation of lower energy. Therefore, for the purpose to progress various photocatalytic reactions effectively, a stable photocatalyst which acts by visible light is needed. However, among the conventional technique, there is no technique to satisfy the above mentioned requirement.
The U.V.(ultra violet).visible light absorption spectrum of cadmium sulfide CdS on the market (99.99% purity) is already measured and it is known that it absorbs the light of spectrum region from ultra violet to visible light of 550 nm. Further, according to the reports of many researchers, in the case of CdS, since a valence electron band and a conduction band, which forms band gap 2.4 eV, has surplus to generate oxygen and hydrogen by electric potential view, theoretically it is deemed to have an ability to decompose water to hydrogen and oxygen. However, there is a report which reports that when decomposing reaction of water is carried out by irradiating visible light over than 440 nm, hydrogen can be obtained stable, but the generation of oxygen can not be observed at all. This phenomenon can be explained as follows. That is, photo dissolution of catalyst itself, in other word, photo-corrosive action is caused because of instability of chemical species on catalyst surface and when a positive hole in the inside of solid generated by photo excitation is transferred to the surface, it oxidizes a S2− on the surface prior to a water molecule and forms a surface
From the above mentioned fact, it is obviously known that the pure sulfide CdS can reduce protons to hydrogen by visible light having longer wavelength than 440 nm, but is not so stable material to generate oxygen from water and does not have such an ability.
The inventors of the present invention have conjectured that since a valence electron of a nitrogen atom has higher energy than that of oxygen atom, a band gap energy of metal compound containing nitrogen atom can be make smaller than that of metal oxide, and considered that a metal and a metal compound bonded with adequate amount of nitrogen atoms become possible to generate excitation electrons and holes and will become a photocatalyst which acts by visible light irradiation. And the inventor of the present invention synthesized oxynitride containing a transition metal and proposed a photocatalyst which acts by visible light (refer to JP Application 2000-256681 published as unexamined application JPA 2002-066333 on May 3, 2002).
The object of the present invention is to provide a novel photocatalyst which acts at broad wave length region of visible light. The inventors of the present invention have further proceeded an idea of above mentioned oxynitride containing transition metal, and think of that a photocatalyst which acts by visible light can be obtained from a compound wherein transition metal is bonded with nitrogen alone. Therefore, the inventors of the present invention have concentrated in the investigation for the synthesis of stable transition metal nitride, and have found out that the compound containing rhombic tantalum nitride of characterizable by chemical formula Ta3N5 can be used in stable as a photocatalyst. Thus the object of the present invention is accomplished. The compound containing rhombic tantalum nitride means that the compound is not necessary to be a pure rhombic tantalum nitride but is sufficient if the diffraction spectrum of tantalum nitride is detected by an X-ray diffraction.